This invention relates to finely divided aluminum powders. More particularly, this invention relates to a method for the deagglomeration of aluminum powders by modifying the surfaces thereof through chemical means in order to reduce their surface energy.
Finally divided aluminum metal particles, especially in the range of from 2 to 5 micrometers tend to agglomerate and cake to such an extent that their free-flow characteristics are severely impaired. The free-flow characteristic of aluminum powders is necessary in order to improve its dispersibility in the industrial processes which find it to be a value. The powder particles tend to agglomerate by weak interaction in order to form clusters which usually exhibit a size greater than 35 micrometers. Interactions of this type arise from non-specific attractive forces which can usually be described in terms of van der Waals forces and electrostatic attraction. The aggregation of aluminum particles may also involve small contributions from metallic as well as hydrogen bonds. These attractive forces which exist between aluminum particles can be described by the following relationship: EQU F=A(d.sub.1 d.sub.2 /d.sub.1 -d.sub.2) (1)
Where A is a constant independent of the dimensions of the spherical particles and d.sub.1 and d.sub.2 represent the diameters of the 2 solid spheres. The letter A encompasses the number of attracting atoms on the spheres, the van der Waals constant, and the distance between the centers of the surface molecules or atoms of the two spheres. Electrostatic forces are considered to be a minimum in this size range. In similar studies, electrostatic contributions were found to be less than 9 percent.
Although molecular or van der Waals forces are generally considered to be weak forces, they can nonetheless be considerable between micrometer sized particles at very short distances. Evidence of this is provided by the observation that aluminum powder dispensed from evacuated canisters exhibits a greater agglomerative tendancy than powder that has not been subjected to vacuum treatment. It was theorized, therefore, that the removal of naturally adsorbed atmospheric gases enhances interparticle attraction by permitting closer contact.
Since it appears that prevention of agglomeration may be a matter of shielding of one particle from another, a research effort was conducted in an attempt to modify the surfaces of the aluminum spheres in order to reduce the attraction between them. As a result, it was found that surfaces modifications could be accomplished by treating the aluminum powders with water followed by air drying or drying in acetone. This procedure improved to a substantial degree the free-flow characteristics of finely divided aluminum metal powders; especially those powders in the micrometer sized range.